In recent years toothed belts have been used in the driving units of various kinds of machines. These toothed belts each have a woven fabric adhesive-bonded to the surface thereof which are adapted to be brought into contact with pulleys for the purpose of protecting the surface. The yarns forming the woven fabric, as for the yarns to be located in the longitudinal direction of the toothed belts, there have been used crimped yarns of synthetic fibers.
FIG. 1 shows a fragmentary perspective view of a toothed belt of the prior art. This toothed belt is constructed such that it comprises a rubber body 4 having tensile cords 3 embedded therein. The rubber body has teeth 6 formed on one surface thereof, and a woven fabric 2 adhesive-bonded to the surface 5 thereof adapted to be brought into contact with pulleys. As shown in FIG. 2, this toothed belt of the prior art is manufactured by locating a woven fabric 2, tensile cords 3 and unvulcanized rubber material 4 on a mold 1 having teeth or recessed and projecting portions on the surface thereof, and pressurizing and vulcanizing them so as to mold them. At that time, as shown in FIG. 3, the surface 5 of the rubber body which is to be brought into contact with pulleys is formed with teeth 6 along the recessed portions of the mold 1. The above-mentioned woven fabric 2 must meet requirements such that it must be elongated in the longitudinal direction of the belt along the teeth or recessed and projecting portions and is capable of elongating by several ten percents when it is subjected to low loading. Accordingly, as for the yarns in the woven fabric 2 extending in the longitudinal direction of the toothed belt, crimped yarns of synthetic fibres have so far been used so that they can be elongated appreciably from their crimped condition.
As for the above-mentioned crimped yarns of synthetic fibers in the woven fabric of the kind specified, there have so far been utilized nylon yarns for clothing. This nylon yarn for clothing has a relative viscosity of about 2.50 to sulphuric acid, a filament strength of 6 g/d and a filament fineness of about 3 deniers.
The reason for the use of the crimped yarns comprising nylon yarns for clothing is that in order to obtain strong adhesion between the woven fabric and the rubber material which forms the body portion of the toothed belt the nylon fibers are selected as the stock or raw material thereof, and only nylon yarns for clothing are generally available in the form of crimped yarns of nylon filaments. The nylon yarns for clothing are, however, inferior in wear resistance and heat resistance, and also the filament strength of such yarns after they have been subjected to crimping becomes as small as 5 g/d or under. Therefore the woven fabric formed by using such yarns has been incapable of protecting satisfactorily the surface of a toothed belt adapted to be brought into contact with pulleys, thus rendering it impossible to provide a satisfatory endurance to the toothed belt.
Furthermore, it should be noted that the inventor of the present invention has previously invented a method of subjecting nylon yarns for clothing for industrial materials, which are generally used for industrial application, to crimping, and filed a patent application with a Japanese Patent Application No. SHO 58-96358. (Refer to Japanese Patent Laid-open Publication No. SHO 59-223335.) The inventor of the present invention also found out that the crimped nylon yarns obtained by the above-mentioned method have a high strength and a sufficient shrinkage percentage, and they can be used satisfactorily to form a woven fabric to be adhesive-bonded to toothed belts. (Refer to Japanese Patent Laid-open Publication No. SHO 59-222639.)
The above-mentioned nylon yarns for industrial materials have a relative viscosity of 2.65 to 2.80 to sulphuric acid, a filament strength of 8 g/d, and a filament fineness of 5 to 10 deniers. By subjecting such nylon yarns to the crimping method of the above-mentioned invention, it is possible to obtain yarns whose filament strength after crimping is not less than 5 g/d and is higher than those of nylon yarns for clothing and whose shrinkage percentage is as high as 50% or over.
Further, in U.S. Pat. No. 4,604,081, there is disclosed a toothed belt formed by using woolly processed nylon yarns for industrial materials having an ultimate viscosity of 1.30 or over (and having a relative viscosity to sulphuric acid of 2.89 or over) for the yarns in the woven fabric to be located in the longitudinal direction of the belt.
The inventor of the present invention has made adhesive-bonding of a woven fabric formed by using crimped yarns obtained by crimping nylon yarns for industrial materials as mentioned above to the surface of a toothed belt adapted to be brought into contact with pulleys, and then carried out endurance tests on the belt. As a result, it was found out that in such a toothed belt the above-mentioned crimped yarns were cut off and cracks occurred in the root and bottom portions of the teeth and the portions in the vicinity thereof.
Since the cracks which occur in the root and bottom portions of the teeth of the belt are caused by the cut-off of the crimped yarns in the woven fabric adhesive-bonded to these portions, the inventor of the present invention utilized initially, aiming at an improvement of this respect, crimped yarns obtained by subjecting to crimping nylon yarns having a relative viscosity of 2.65 to 2.80 to sulphuric acid, a filament fineness of less than 5 d, and a filament strength of 5 g/d or over in terms of performance, for the yarns to be located longitudinally in a toothed belt so as to form a woven fabric, and provided a toothed belt having a woven fabric adhesive-bonded thereto. (Japanese Utility Model Laid-Open Publication No. SHO 61-96035).
However, since nylon yarns for industrial materials having a filament fineness of less than 5 deniers are not available in general, the nylon yarns as mentioned above need to be produced specially by melt spinning, and even when such nylon yarns were produced, it was difficult to subject them to stretching. Accordingly, difficulties arose in that yarns having a uniform filament fineness could not be obtained.